Escherichia coli

 Introduction:  Escherichia coli is a common bacterial cause of diarrhea in young farm animals.  Usually, other agents complicate the disease process, but E. coli can be the sole causative agent of disease.  

Several types of E. coli cause problems.  These bacteria are categorized based on the toxins produced and the pathology resulting from these specific toxins.  The categories include enterotoxigenic, enterohemorrhagic, enteropathogenic, enteroadherent, and enteroinvasive strains.  Enterotoxigenic E. coli (ETEC) is the most common cause of diarrhea in neonates and produce a disease called colibacillosis or "yellow scours."  However, enterohemorrhagic E. coli (EHEC) appears to be emerging as another common cause of diarrhea in calves.


Pathophysiology:  

Type of E. coli Toxin Produced Pathology Caused
ETEC heat labile enterotoxin

and

heat stable enterotoxin

Pathogenic due to fimbria or pili that allow ETEC to attach to enterocytes.  K99, K88, and F41 antigens allow for binding (which is required).

Release an enterotoxin that stimulates secretion of water, sodium, and chloride. This disturbs the electrolyte balance in the intestine, causing a secretory diarrhea

No gross or histologic lesions produced.
EHEC shiga-like toxin

and

verotoxin

Bacteria primarily colonize the colon.

Produce an attaching and effacing lesion on the intestinal mucosa, primarily the distal small intestine and colon.

Verotoxin causes vascular damage resulting in bloody diarrhea.


Signalment:  Calves with diarrhea due to enterotoxigenic E. coli are almost invariably less than 2 weeks of age and most commonly less than 4-5 days old.  Co-infection with another pathogen may extend the age range somewhat.  Enterohemorrhagic E. coli can also cause diarrhea in older calves.

Clinical Signs:  Clinical signs of ETEC infection include profuse, watery diarrhea (aka "white scours"), depression, weakness, dehydration, anorexia, and shock resulting from hypovolemia and electrolyte imbalances.  Mild cases may recover spontaneously, while severe cases my result in death in as little as 8 hours (sometimes without external evidence of diarrhea).  The signs of an EHEC infection are similar with the addition of the presence of blood in the diarrhea.


Gross Pathology:  In an ETEC infection, fluid filled loops of bowel may be seen on necropsy, but because the bacteria does not invade into the mucosa, there are no gross lesions.   EHEC may produce bloody intestinal contents.  With EHEC, pathologists will sometimes see small mucosal foci of hemorrhage or erosion in the colon and/or distal small intestine, but this is also an inconsistent finding.

 


Histopathology:  Again, because ETEC does not invade the mucosa, there are no histological lesions.  On occasions and with optimal fixation, the bacteria may be seen lining the epithelial cells of the intestine (see figure).

Right: This photomicrograph shows ETEC adhered diffusely to the luminal surface of enterocytes (arrows).  More importantly, note that enterocytes are intact.  There is neither significant mucosal damage or inflammation.

 

EHEC causes mucosal damage, specifically an attaching and effacing lesion found most commonly in the colon. 

 

Diagnostic Procedures:  

Clinical signs and history, combined with the age of the animal, are helpful in determining whether E. coli is the cause of illness.  An ELISA test which detects the presence of pilar antigens can be run on feces to determine if ETEC is the causative agent.  A verotoxin assay performed on feces or a chilled colon sample determines whether EHEC is the causative agent.  
Remember that non-pathogenic strains of E. coli are normal flora and will often be isolated from feces.  
Once an E. coli is isolated from diarrheic calf feces, the Washington Animal Disease Diagnostic Laboratory (WADDL) is using new a multiplex PCR method to identify enterotoxigenic and enterohemorrhagic strains based on the presence of virulence genes which are not present in normal flora.  See fecal culture for more information on how results are reported by WADDL.